Windows only: While Outlook 2007 has a built-in feed reader, those of us still using 2003 or even Outlook Express (d'oh) are up a creek—without an add-on, that is. The Pain in the Tech blog recommends RSS Popper:

After installation RSS feeds are displayed under a folder called RSS in the "All Mail Folders" pane on the left-side navigation. The keyboard shortcuts are to hit "Ctrl-Y" to bring up the Go To Folder menu, then hit "R" for RSS and enter.

RSS Popper also supports private feeds that require a username and password; one good reason to use a desktop reader versus an online reader like Google Reader or Bloglines. Unlike most of our featured downloads, I wasn't able to test RSS Popper myself (no copy of Outlook 2003 readily available), so proceed with caution. RSS Popper is freeware for Windows only.